1. Field of the Invention
The present invention is generally related to hydrostatic pressure liquid level sensors, and more particularly to a method and device for the automated measurement of the levels of liquids dispensed into processing tanks, such as those utilized in the manufacture of semiconductor wafers.
2. Brief Description of the Prior Art
The processing of semiconductor wafers typically involves several steps in which the wafers are immersed in various corrosive liquid baths to etch material from the surface of the wafers. The baths in which the etching process occurs are typically small tanks which hold two to five gallons of etchant liquid. Such etching processes have become automated to improve quality control and increase throughput. The automated etching processing equipment requires liquid level sensors to control the quantity of various liquids that are automatically dispensed into the tank, such that the proper concentrations of the etchant liquids are automatically established.
A variety of liquid level sensors have been utilized to sense the liquid level in a tank and control the flow of liquids into the tank. One such device utilizes a hollow tube that is inserted downwardly through the surface of the liquid into the tank. A pressure sensor is engaged to the outward end of the tube to detect changes in the air pressure within the tube as a measure of differing liquid levels within the tank which change the air pressure within the tube. A disadvantage of such devices is that they involve a physical intrusion into the tank which can obstruct the automated equipment, and with etchant liquid components such as hydrogen peroxide, additional air bubbles may enter the tube and cause erroneous readings. Other liquid level sensors, such as floats, are also intrusive into the work area and therefore are disadvantageous.
Another type of liquid level detector utilizes a hollow L-shaped tube which projects from the bottom of the tank outwardly and upwardly. The liquid level in the upwardly projecting portion of the hollow tube mirrors the liquid level in the tank and thus is an indication of the level in the tank. A plurality of optical detectors are positioned at particular level locations along the length of the upwardly projecting portion of the tube, such that the passage of the liquid level within the tube through such an optical detector is an indication of the liquid level within the tank. This type of level detector becomes inaccurate when liquids having significantly different specific gravities are sequentially input into the tank. For instance, if the tank is initially partially filled with a low specific gravity liquid such as water, and then a high specific gravity liquid such as concentrated sulfuric acid is added, the high specific gravity liquid will flow to the bottom of the tank and into the inlet of the exterior tube. The denser liquid will then cause erroneous readings within the measurement tube. Also, the manual adjustment of each optical detector is required in order to utilize the tank for a different task.
Other liquid level measurement devices include ultrasonic detectors which operate on significantly different principles from the present invention.